Conventionally, in order to address multiplication of power sources of overhead lines, auxiliary power supplies for train cars have been designed to supply a 400V class three-phase AC voltage as power to an air-conditioning apparatus. In one such power supply, in order to reduce weight and save space of the auxiliary power supply side, an isolation transformer of an output unit is omitted and non-isolated power is supplied. In this case, the power supplied from the auxiliary power supply has a waveform with a 600V class square wave as a peak value. Since the air-conditioning apparatus is equipped at a position apart from the auxiliary power supply, an equipment line for power provided from the auxiliary power supply to the air-conditioning apparatus has a length of 40 m or more in a long one; accordingly, it is known that a peak value of the waveform of the power supplied to the air-conditioning apparatus may become twice or more of the 600V class (peak value). This may accelerate dielectric breakdown of the winding of the electric motor used in the air-conditioning apparatus and may bring about an adverse effect on the life of the electric motor.
There is a known air-conditioner power supply system that accommodates to two power sources (AC power source/DC power source) by providing an AC power source input terminal and a DC power source input terminal on the input side of an inverter device built-in into the air-conditioning apparatus. In this case, when power supplied to the air-conditioning apparatus is an AC voltage, the power is supplied to the electric motor of the air-conditioning apparatus from the inverter device via a rectifier unit that is built-in in the air-conditioning apparatus. Furthermore, when the power is of a DC voltage, the power is supplied to the electric motor inside the air-conditioning apparatus from the inverter device; hence, there will be no case in which the peak value of the waveform of the power becomes twice or more of the 600V class (peak value), and the effect on the life of the winding of the electric motor becomes small (see Patent Literature 1, for example).